The Texas guide to rainwater harvesting has a bunch of information on cisterns including build-your-own options: http://www.twdb.state.tx.us/publications/reports/RainwaterHarvestingManual_3rdedition.pdf

Another resource is: http://www.sustainablebuildingcentre.com/forumtopic/rainwater_collection_systems

Some preliminary research shows that it is not a simple answer. Here are some datapoints, but not a real answer yet:

Zero net energy buildings provide a technically feasible approach to reducing energy consumption in buildings. These buildings are designed and constructed to generate all of the energy they require through a combination of energy efficiency and renewable energy generation technologies. Viable renewable energy sources consist of photovoltaic cells, solar water heaters, and geothermal heat pumps.

The Palo Alto Research Center (PARC), formerly the birthplace of much of the computer and communications innovation over the past 10 years or so has now realized the massive opportunties in connecting Cleantech innovation to building.

On March 20, 2007, Environment Canada, Pacific and Yukon Region, and the British Columbia Environment Industry Association are hosting a workshop on Funding for Environmental Technology Innovation.

In both the Vancouver Sun and the Globe and Mail today, there are full sections on the environment, climate change, and going green. It's incredible. Are we approaching critical mass?